RELATIONS between Britain and Russia have cooled to a level not seen since the Cold War as Prime Minister Theresa May expelled 23 diplomats, severed high-level contacts and vowed open and covert action against Kremlin meddling after the poisoning of a former spy.

Russia said it would respond soon to what it called Britain's "crude" and "hostile" actions.

While Mrs May pledged to disrupt Russian espionage and "hostile state activity," she gave few details about how hard Britain would hit Russian politicians and oligarchs where it really hurts - in their wallets.

She vowed further action against Russia after a deadline passed for the nation to confirm its involvement in the attempted murder of former spy Sergei Skripal and his daughter Yulia with a nerve agent. Picture: Christopher Furlong/Getty Images

Mrs May told the House of Commons that 23 Russians diplomats who have been identified as undeclared intelligence officers have a week to leave Britain. "This will be the single biggest expulsion for over 30 years," she said, adding that it would "fundamentally degrade Russian intelligence capability in the UK for years to come."

The Prime Minister spoke after Moscow ignored a midnight deadline to explain how the nerve agent Novichok, developed by the Soviet Union, was used against Sergei Skripal, an ex-Russian agent convicted of spying for Britain, and his daughter Yulia. The pair remain in critical condition in a hospital in Salisbury, southwestern England, after being found unconscious on March 4.

Mrs May said "there is no alternative conclusion other than that the Russian state was culpable for the attempted murder of Mr. Skripal and his daughter."

She announced a range of economic and diplomatic measures, including the suspension of high-level contacts with Russia. An invitation for Russian Foreign Minister Sergey Lavrov to visit Britain has been cancelled, and British ministers and royals will not attend the soccer World Cup in Russia this summer. Mrs May also said Britain would clamp down on murky Russian money and strengthen its powers to impose sanctions on abusers of human rights, though she gave few details.

Relations between the nations are at a new low after the attack on Mr Skripal, who was granted refuge in the UK following a ‘spy swap’ between the US and Russia. Picture: Christopher Furlong/Getty Images

"We will freeze Russian state assets wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents," said Mrs May, promising to use all legal powers against criminals and corrupt elites, and to "increase checks on private flights, customs and freight."

Mrs May said some of the measures "cannot be shared publicly for reasons of national security."

"There is no place for these people - or their money - in our country," she said.

The Russian Embassy in London said the expulsion of diplomats was "totally unacceptable, unjustified and shortsighted."

The official Twitter account for the Russian Embassy hit back with a cheeky tweet referencing talk of a new Cold War.

Russia did not immediately announce retaliatory measures, but its Foreign Ministry said "our response will not be long in coming."

It said Britain's "hostile measures" were "an unprecedentedly crude provocation." Some Russia experts said the measures announced by Mrs May were unlikely to make Russian President Vladimir Putin's government change its behaviour. She did not expel Russia's ambassador or announce sanctions against any individuals or companies.

Critics of the British government have long claimed that the UK is reluctant to act against Russia because London's property market and financial sector are magnets for billions in Russian money.

"There does not seem to be any real appetite so far to investigate the ill-gotten gains of the Russian elite that have been laundered through London," said John Lough, an associate fellow in the Eurasia program at the Chatham House think-tank. "It is not clear to me that London's response will hit the Kremlin where it hurts."

Moscow has denied responsibility for Mr Skripal's poisoning. It refused to comply with Britain's demand for an explanation, saying the UK must first provide samples of the poison collected by investigators.

Some in Russia have suggested that the nerve agent could have come from another former Soviet country.

Politician Vladimir Gutenev, a member of Russia's state commission for chemical disarmament, said Russia had scrapped its stockpile of Novichok. "It is hard to say what may be happening in neighbouring countries," he was quoted as saying by the Interfax news agency.

Britain is seeking support from allies in the European Union and NATO in response to the use of an illegal chemical weapon on British soil. May's office said President Donald Trump told the prime minister the US was "with the UK all the way." But Britain faces an uphill battle in rallying international backing for any new measures against Moscow.

European Council President Donald Tusk said he would put the attack on the agenda at an EU summit meeting next week.

The UN Security Council - of which Russia is a veto-wielding member - was due to meet later on Wednesday at Britain's request to discuss the investigation. NATO promised to help investigate what it called "the first offensive use of a nerve agent" in Europe or North America since the military alliance was founded in 1949.

But it's unclear what, if anything, NATO can do to put more pressure on Russia. Relations between the old Cold War foes are already poor and short of military action the alliance has little leverage.

Mrs May said Russia's use of a chemical weapon was "an affront to the prohibition on the use of chemical weapons. And it is an affront to the rules-based system on which we and our international partners depend.

"We will work with our allies and partners to confront such actions wherever they threaten our security, at home and abroad."

British Prime Minister Theresa May has dramatically announced the explusion of 23 Russian diplomats from the UK. Picture: Jack Taylor/Getty Images